JMO, there's no real hard data one way or the other on velocity vs. pitching from a slide step that I know of. I've seen the velocity lost in a couple HS pitchers but that is way too small of a sample. Sometimes pitchers who slide step stay closed better and that makes up for the added momentum of the leg lift. A lot of differences get lost in individual mechanics.
I have heard of, but haven't read, studies that showed that rotation had a greater effect than linear momentum and that would then imply that pitchers can get more velocity from a leg lift since almost every pitcher who lifts the leg has some component of rotation, while the rotational momentum isn't going to be there with a slide step to speak of. I suppose a rotational slide step could be done without lifting the leg significantly by starting with the stride foot in a closed location.
In other words, I don't believe the lift directly adds much if anything to velocity but it does help to establish the rotational momentum by moving the cg of that leg to the side. Lifting the leg will move the body's cg upward a bit which will help to create forward momentum if done properly but I don't think it is a big effect relative to how much the lateral cg changes. Chapman seems to use his rotational momentum very effectively and I find it hard to believe he could generate anywhere near the same velocity with a slide step.